Bringing back a LOOKUP value from a table using text field and date ranges

Hi, Will_Rayner 

You don't need to use .date format in code.

I modify your code, Like this:

SLA Target =
MAXX (
    FILTER (
        ALL ( 'sla' ),
        [SLA Code] = SELECTEDVALUE ( 'UW Input for SLA'[SLA Type Name] )
            && [Effective From] <= SELECTEDVALUE ( 'UW Input for SLA'[END_DATE] )
            && [Effective To] >= SELECTEDVALUE ( 'UW Input for SLA'[END_DATE] )
    ),
    [SLA Value]
)

Did I answer your question ? Please mark my reply as solution. Thank you very much.
If not, please upload some insensitive data samples and expected output.

Best Regards,
Community Support Team _ Janey